ARI's offer of $20 per share is a premium of 5.4% to Greenbrier's stock price of $18.97 at the close of trading Monday on the New York Stock Exchange. The shares were up 3% at $19.50 before the bell Tuesday.

Greenbrier's shares have risen 36% since Icahn reported a 9.99% stake in the company last month, making him the company's largest shareholder.

The proposed acquisition is not Icahn's first effort; the investor attempted to merge the two companies in 2008, but failed to complete the merger due to unspecified complications.

Icahn Enterprises LP controls 55.6% of St. Charles, Mo.-based ARI, according to a recent filing.